Manifold Absolute Pressure (MAP) is calibrated to measure absolute pressure. The bellows are connected to the engine inlet manifold, measuring the pressure behind the throttle valve, which is below atmospheric pressure during the operation of the engine and which is higher with a higher engine power setting. As long as there is no supercharger, the MAP should be less than atmospheric during operation.

A manifold pressure value equal to the atmospheric value would only be obtained if the engine was turned off. If measured during engine operation, this would be an indication of a leak in the pressure line that was allowing atmospheric air to enter the line and reach the sensor.